1. Field of the lnvention
This invention relates to a fiber reinforced plastic sheet having a gradient layer structure wherein the number of reinforcing fibers continuously varies, and a process for producing the same.
2. Description of the Prior Art
Heretofore, various plastic sheets composed of thermoplastic resins and reinforcing fibers have been developed. They can be divided into the following groups. One group is produced by mixing reinforcing fibers such as glass fibers with a thermoplastic resin in a melted state by a screw extruder, and extruding the blend in a sheet form through a T die. Another group involves a laminate of a mat of continuous fibers such as glass fibers and a fiber sheet (Japanese Patent KOKOKU No. 54-36193). A further group is a laminate of a chopped strand mat of a surfacing mat of discontinuous fibers and a resin sheet (Japanese Patent KOKOKU No. 51-14557). Another group is produced by mixing chopped strands such as glass fibers, with a resin powder with water in a slurry, then dehydrating, drying, and pressing under heating to form a sheet (Japanese Patent KOKAI No. 60-158228). Yet another group is produced by opening and mixing chopped strands with a resin powder in a gaseous phase, and pressing under heating the cotton-shaped mixture to form a sheet (Japanese Patent KOKOKU No. 64-6227, Japanese Patent KOKAI No. 59-49929). Finally another group is produced by mixing a resin powder with unopened chopped strands, and pressing under heating to form a sheet (Japanese Patent KOKOKU No. 51-20550).
However, in the first method using an extruder, since most of the reinforcing fibers are cut to less than 1 mm in length by the screw of the extruder, the reinforcing effect of the thermoplastic resin, particularly impact strength, is insufficient. The second method of laminating a mat of continuous fibers is a most common method. In this method, since the fibers are continuous, the fluidity of the fibers is insufficient through its molding process such as compression molding using a mold into an arbitrary form under heating. Therefore, when a complex form such as a rib is molded, the filled amount of the fibers in the rib is low. The third method of laminating a chopped strand mat or surfacing mat of discontinuous fibers is expensive because of using the mat as a raw material for the sheet. Moreover, the fluidity of the fibers is inferior through its forming process into a sheet, because of binding the fibers by coating a binder thereto in order to keep the mat form of the chopped strand mat or the surfacing mat. In the method of mixing chopped strands with a resin powder with water in a slurry, the chopped strands are opened in water into monofilaments. Therefore, discontinuous monofilaments of about 7 to 50 mm in length are uniformly dispersed in the plastic sheet, and, as a result, the molded articles of the sheet formed into arbitrary forms are excellent in appearance. Moreover, this method is excellent from the viewpoint that the fibers are filled in complex form portions. However, since monofilaments are entangled with each other in the sheet, during molding a great flow resistance occurs to flow of the fibers into an arbitrary form together with a resin. As a result, great pressure is necessary for the molding. Moreover, short shot occasionally occurs because of the problem of resin fluidity. Another problem of this method of mixing chopped strands in water is low impact strength, due to the monofilaments. In view of its manufacturing process, the manufacturing cost is increased by the dehydration process of a great quantity of water and the drying process. In the method of opening chopped strands in a gaseous phase, discontinuous fibers and a resin powder are used similarly to the above method, but it is different in the mixing process which is carried out in a gaseous phase. Therefore, the dehydration process and the drying process do not necessarily result in decreasing the number of process steps. However, since opened strands are dispersed in a plastic resin, the plastic sheet has similar defects to the plastic sheet of the above method, i.e., great flow resistance, great molding pressure, small impact strength, etc. Besides, opened monofilaments are pill shaped, and the monofilaments are oriented at random in three-dimensions in the mixture of the resin powder and the fibers. Therefore, when a sheet formed of the mixture is heated at a temperature higher than the melting point of the resin during molding, the sheet expands due to springback of the fibers, and some fibers are projected from the surface of the sheet to degrade the appearance. In the last method of mixing a resin powder with unopened chopped strands, since unopened-strands are present around the surface of the sheet, the appearance of the molded article is inferior.